1. Field of the Invention
The present invention relates to a helical coil type steam generator, and more specifically to a heat transfer tube, its leak detection unit and a steam generator for a sodium-cooled fast reactor, whereby the heat transfer efficiency of the heat transfer tube is improved, and it is possible to detect on-line, and in real-time, whether or not the heat transfer tube is damaged, and to alleviate a pressure rise in the steam generator when the heat transfer tube is damaged.
2. Description of the Related Art
Currently, research is under way into a reactor that can breed more than nuclear fuel than it consumes while generating power, but few such reactors have been put to practical use for power generation. Plants for power generation that are being researched with a goal of being put to practical use include a fast reactor (a fast breeding reactor or a liquid metal reactor) which uses uranium and plutonium as nuclear fuel, and liquid metal sodium as coolant, without using a moderator but using nuclear fission of plutonium by fast neutron reactions.
The steam generator for a sodium-cooled fast reactor (SFR) using liquid-state sodium as coolant has an advantage in that the cooling system can be operated at a high temperature and low pressure, since the heat from the low pressure and high temperature sodium flowing on the side of the outer shell of the heat transfer tube is transmitted to the high-pressure and low-temperature water/steam flowing in the heat transfer tube.
A number of problems can arise from a conventional steam generator for a fast reactor:
First, single-wall tubes are mainly used as the heat transfer tube of the conventional steam generator for the fast reactor, but double-wall tubes are also used to prevent the danger of sodium and water leaks. In the case of the double-wall tubes, if gas fills a space between an inner tube and an outer tube, the heat transfer efficiency is lowered, so it is important to keep them in close contact to prevent this. But in conventional double-wall tubes, the inner tube and the outer tube are made of the same material, so the degree of heat expansion is different between the outer tube that is in contact with high-temperature sodium and the inner tube that is in contact with water of a relatively low temperature. Therefore, there is a problem when the degree of close contact between the inner tube and the outer tube is lowered, and heat transfer efficiency is lowered. In particular, the problem of low heat transfer efficiency becomes more serious as a temperature difference between the inner tube and the outer tube increases.
Also, if cracks occur because the heat transfer tube is damaged due to corrosion of the heat transfer tube or thermal imbalance in the steam generator, the high-pressure water or steam leaking through the cracked portion comes into contact with sodium, causing a chemical reaction. As a result, a high-temperature corrosive reaction product and a large quantity of hydrogen gas are generated, and the system could be damaged.
Methods for detecting such damage to the heat transfer tube are known. As one of the methods for detecting the damage to the heat transfer tube with double-wall tubes, a fine groove is formed lengthwise between the inner tube and the outer tube, and whether or not the heat transfer tube is damaged is detected by using pressure change of the helium gas injected into this groove. Such damage detection methods using helium gas include a method in which shared helium plenum is formed in the steam generator and the composition of gas is analyzed or the pressure change in the shared helium plenum is measured, and a method in which the composition of helium gas is analyzed outside the steam generator.
But the former has a problem that when the inner tube is broken the relevant heat transfer tube is closed to shut off leakage but if the outer tube is broken it is not possible to isolate the broken heat transfer tube separately so sodium flows into the shared helium plenum to cause a loss of the damage detection function. And the latter also has a problem that if both the inner tube and the outer tube were broken the heat transfer tubes can be isolated, but since all heat transfer tubes are exposed outside the probability of causing a serious accident such as the heat transfer tube being fractured is increasing. Therefore, there is a need to develop a means or method of detecting the damage to the heat transfer tube of the steam generator for a fast reactor that can detect the breakdown of the heat transfer tube on-line at all times.
In addition, a steam generator has a free liquid surface formed on the top for sodium and cover gas to contact each other. But as the free liquid surface is fluctuating, the surface temperature of the heat transfer tube fluctuates due to the high-temperature sodium and the cover gas of relatively low-temperature, which becomes a cause for generating thermal fatigue of the heat transfer tube. Especially in the conventional steam generator, the space of free liquid surface is wide, so the amplitude of fluctuation is large, and due to this the thermal fatigue of the heat transfer tube is high.